Niobium alloys



United States Patent 3,206,305 NIOBIUM ALLOYS Richard T. Begley, Verona, Raymond W. Buckman, In, and Robert L. Ammon, Baldwin Bore, Pa., assignors to Westinghouse Electric tlorporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Feb. 25, 1963, Ser. No. 260,897 9 Claims. (Cl. 75-474) This invention relates to niobium alloys having high strength at elevated temperatures and characterized by being readily hot or cold worked or wrought into desired bars, strips, sheets and the like shapes.

Considerable effort has been expended heretofore toward the development of niobium base alloys having high strength at elevated temperatures for use as a structural material in high temperature engines and other applications. For the most part, these alloys are made by alloying niobium with combinations of tungsten, molybdenum, zirconium, and tantalum. While these alloys are generally characterized by good strength at elevated temperatures they are also generally marked by poor workability especially at lower temperatures thus requiring considerable efiort to fabricate them into desired shapes. As an illustration, niobium has been alloyed with 15% tungsten, molybdenum, and 1% zirconium. While this alloy exhibits good strength characteristics at elevated temperatures, 57,200 psi yield (.2% offset) and 63,300 p.s.i. ultimate tensile strength at 2000 F., it is marked by workability characteristics rendering it extremely difficult, if not impossible, to fabricate desired shapes at room temperature.

An object of this invention is to provide niobium alloys characterized by high strength at elevated temperatures and good hot and cold working properties.

Other objects of this invention will in part be obvious and will in part appear hereinafter.

The novel niobium base alloys of the present invention are characterized by high strength at elevated temperatures and good hot and cold working properties. Such alloys consist essentially of from 20% to 40% by weight of tantalum, from 2% to 20% by weight of vanadium, up to about 5 equivalent percent by weight of at least one element from the group consisting of zirconium and hafnium, such equivalent percent being determined by the equation: zirconium percent plus /2 hafnium percent equals equivalent percent, and the balance being niobium, and incidental impurities. Particularly good results are had with alloys consisting essentially of about 25%, for example, 23% to 28% tantalum, 3% to 7% vanadium, with or without small amounts, for example 1% to 2%, of zirconium, or equivalent hafnium, and the balance being niobium, and incidental impurities.

It will be understood that in practicing this invention normal zirconium, containing up to and even exceeding 3% hafnium can be employed. Further, hafnium may be freely substituted for zirconium. However, since the atomic percent of the various constituents rather than the weight percent is of principal importance, the latter being used principally for convenience purposes, the equation indicated above will govern the relative amounts of hafnium and zirconium. That is, in substituting hafnium for zirconium, the equivalent atomic ratio is provided by adding twice the Weight of hafnium as the zirconium it replaces, since an atom of hafnium weighs 3,206,305 Patented Sept. 14, 1965 "ice approximately twice as much as one of zirconium. It will also be understood that the metal niobium is sometimes called columbium.

The alloy of this invention is preferably prepared by the double consumable arc process carried out in a vacuum. Either electric are or electron beam melting may be employed. The resulting ingot is heated to around 2000 F. and forged to effect a reduction of at least 25 into a slab or plate to break down the crystal structure. This slab or plate is then recrystallized by heating to 2500 F. to 2700 F. for about 1 hour and then may be hammered, rolled or otherwise processed at room temperature into sheets, bars, strips or other shapes. It will be appreciated that other methods of producing the alloy may be employed, the method described being principally for illustrative purposes. For example, other melting means may be employed, however the double consumable electrode melt is preferred, at present, since it is apparently superior from the stan point of homogeneity of the resulting ingot.

Basically, the alloy of this invention consists of the ternary system of 20% to 40% tantalum, 2% to 20% vanadium, the balance being niobium. This alloy is characterized by high strength at elevated temperatures and very good workability properties at room temperature.

EXAMPLE I An alloy consisting of 25 tantalum, 5% vanadium, oxygen, nitrogen and carbon combined being less than 0.06%, the balance being niobium was prepared by the arc melting method and the resulting ingot hotforged at 2000 F. into a thin slab or plate. This slab or plate was then cold rolled at room temperature to elfect a reduction resulting in a sheet .050 inch in thickness. Tensile tests on samples from this sheet gave the results listed in Table I:

Table I 2% offset Ultimate Temp. yield tensile strength, strength, p.s.i. p.s.i.

Further it has been found that the addition of zirconium, in small amounts, further strengthens the alloy of this invention without any serious decrease in workability. The strengthening efiect of zirconium. is even more pronounced in the presence of small amounts of the interstitial impurity elements carbon, nitrogen and oxygen. The strengthening mechanism here appears to be the formation of dispersed phases based on zirconium oxides, nitrides and carbides. It is to be understood that this theory appears to satisfactorily explain this phenomenon but the invention is to be in no way restricted thereby.

EXAMPLE II An alloy consisting of 25% tantalum, 5% vanadium, 1% zirconium, oxygen, nitrogen and carbon combined being less than 06%, the balance being niobium was prepared by the method descirbed above in Example I Table II 2% set Ultimate Temp. yield tensile strength strength p.s.i. p.s.i.

Roon1 75, 200 05, 800 2,000 F. 45, 700 52, 700 2,400 F 19, 200 23, 900

The addition of .05 oxygen to the alloy described in Example II increased the 2000 F. yield and ultimate tensile strength value to 55,700 p.s.i. and 58,150 p.s.i. respectively. It can be readily seen that these mechanical property values compare quite favorably to the values for the 15% tungsten, 5% molybdenum, 1% zirconium alloy mentioned earlier.

Further, the alloys of this invention are characterized by very good cold Working properties foreign to other niobium base high temperature alloys. Tensile tests on the alloys of this invention indicated that their uniform elongation, approximately 18%, is essentially identical with that of pure niobium, which is characterized by extremely good room temperature workability properties. It should be noted that pure niobium, however, has very poor high temperature strength. It should also be noted that. in contrast with the good room temperature uniformelongation characteristics of the alloys of the present invention, other high temperature niobium base alloys generally exhibit extremely low or nil uniform elongation under similar conditions of testing. Further, the extreme, 65%, reduction by cold rolling mentioned above in Examples I and II, performed on the alloys of the present invention could not be performed on most current niobium base alloys of equivalent high temperature strength without severe cracking.

The precise action or mechanism of the interstitial impurity elements carbon, nitrogen and oxygen in the alloys of this invention is unknown. However, it is known that they seriously curtail workability if present in excessive amounts, and therefore their presence in the alloy must be limited. Further, it has been found that the addition of zirconium, or hafnium in small amounts, in addition to adding to the strength of the alloy, minimizes the adverse effects of the interstitial impurity elements on workability. Thus, if the zirconium content is less than about /2 the combined intertitial element content should be kept below about .15 On the other hand, if the zirconium content exceeds about /2 the permissible interstitial element content may be slightly higher, that is up to about .25%.

In practicing the present invention, it is well to bear in mind that while zirconium is an effective strengthener, its presence may curtail workability. Hence, where maximum low temperature workability is of critical concern, it is recommended that the equivalent zirconium content be restricted to a maximum of 2%. Of course, if the equivalent zirconium content is less than /2% the intertitial impurity elements should be present in an amount of no more than .15%. Also, since the presence of tantalum increases the density of the alloys of this invention, tantalum should be limited somewhat, for example, 20% to 30%, where lightness of Weight is of critical importance.

Other examples of suitable alloys are:

Tantalum 30%.

Vanadium Zirconium 0.4%.

Tantalum Vanadium 8%. Zirconium 1.0%. Oxygen, nitrogen and carbon combined Less than .15 Niobium Balance. Oxygen, nitrogen and carbon combined Less than .25 Niobium Balance.

It will be appreciated that numerous uses may be made of the alloys prepared in accordance with the present invention. It is intended that the disclosure be construed as illustrative of the invention and not in limitation thereof.

What is claimed is:

1. An alloy consisting of from 20% to by Weight of tantalum, from 2% to 20% by weight of vanadium, up to about 5 equivalent percent by weight of at least one element from the group consisting of zirconium and hafnium, said equivalent percent being determined by the equation: zirconium percent plus /2 hafnium percent equals equivalent percent, the combined total of carbon, oxygen and nitrogen not exceeding .25% and, further, not exceeding .15 Where the combined total of Zirconium and hafnium in accordance with the above said equation is less than .05% by weight, and the balance being niobium.

2. A member comprising a cold worked and shaped body of alloy conisting of from 20% to 40%, by weight of tantalum, from 2% to 20% by weight of vanadium,

up to a maximum of 2 equivalent percent by weight of at least one element from the group consisting of zirconium and hafnium, said equivalent percent being determined by the equation: zirconium percent plus one-half hafnium percent equals equivalent percent, the combined total of carbon, oxygen and nitrogen not exceeding 25% and, further, not exceeding .15 Where they combined total of zirconium and hafnium in accordance with the above said equation is less than .5 by weight, and the balance being niobium, the member having high strength at temperatures up to 2400 F. and good workability characteristics at room temperature.

3. An alloy consisting of from 20% to 30% by weight of tantalum, from 2% to 20% by weight of vanadium, up to about 5 equivalent percent by weight of at least one element from the group consisting of zirconium and hafnium, said equivalent percent being determined by the equation: zirconium percent plus one-half halfnium percent equals equivalent percent, the combined total of carbon, oxygen andnitrogen not exceeding .25 and, further, not exceeding .15% Where the combined total of zirconium and hafnium in accordance with the above said equation is less than .5% by weight, and the balance being niobium.

4. An alloy consisting of about 25% by weight of tantalum, 3 to 7% by weight of vanadium, from /2 to 3 equivalent percent by weight of at least one element from the group consisting of zirconium and hafnium, said equivalent percent being determined by the equation: zirconium percent plus /2 hafnium percent equals equivalent percent, the combined total of carbon, oxygen and nitrogen not exceeding .25 and the balance being niobium.

5. A member comprising a worked and shaped body of alloy consisting of about. 23% to 28% by weight of tantalum, from 3% to 7% by weight ofvanadium, from /2 to 3 equivalent percent, by Weight of at least one element from the group consisting of zirconium and hafnium, said equivalent percent being determined by the equation: zirconium percent plus /2 hafnium percent equals equivalent percent, the combined total of carbon, oxygen and nitrogen not exceeding .25 and the balance being niobium.

6. An alloy consisting of from 20% to 40% by Weight of tantalum, from 2% to 20% by Weight of vanadium, the combined total of carbon, oxygen and nitrogen not exceeding .15 and the balance being niobium.

7. An alloy consisting of from 20% to 30% by weight of tantalum, from 2% to 20% by Weight of vanadium, the combined total of carbon, oxygen and nitrogen not exceeding .15%, and the balance being niobium.

8. An alloy consisting of about 25% by Weight of tantalum, from 3% to 7% by Weight of vanadium, the com bined total of carbon, oxygen, and nitrogen not exceeding .15% and the balance being niobium.

9. A member comprising 21 Worked and shaped body of alloy consisting of about 23% to 28% by weight of tantalum, from 3% to 7% by weight of vanadium, the

References Cited by the Examiner UNITED STATES PATENTS 2,822,268 2/58 Hix 75-l74 3,012,883 12/61 Allen 75l74 3,027,255 3/62 Begley 75174 3,028,236 4/62 Wlodek 75174 DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. 

1. AN ALLOY CONSISTING OF FROM 20% TO 40% BY WEIGHT OF TANTALUM, FROM 2% TO 20% BY WEIGHT OF VANADIUM, UP TO ABOUT 5 EQUIVALENT PERCENT BY WEIGHT OF AT LEAST ONE ELEMENT FROM THE GROUP CONSISTING OF ZIRCONIUM AND HAFNIUM, SAID EQUIVALENT PERCENT BEING DETERMINED BY THE EQUATION: ZIRCONIUM PERCENT PLUS 1/2 HAFNIUM PERCENT EQUALS EQUIVALENT PERCENT, THE COMBINED TOTAL OF CARBON, OXYGEN AND NITROGEN NOT EXCEEDING .25% AND FURTHER, NOT EXCEEDING .15% WHERE THE COMBINED TOTAL OF ZIRCONIUM AND HAFNIUM IN ACCORDANCE WITH THE ABOVE SAID EQUATION IS LESS THAN .05% BY WEIGHT, AND THE BALANCE BEING NIOBIUM. 